This invention relates to the redistribution of polycarbonates, and more particularly to a redistribution method which is capable of producing a polycarbonate with a low volatiles level.
Polycarbonates have traditionally been prepared by an interfacial method involving the reaction of a dihydroxyaromatic compound with phosgene, or a melt method in which the phosgene is replaced by a diaryl carbonate. In recent years, however, a redistribution process for use with polycarbonates has been developed. This process, described in U.S. Pat. Nos. 5,414,057 and 5,459,226, comprises heating an already formed polycarbonate in the presence of a redistribution catalyst to produce a polycarbonate with a different, generally lower, molecular weight. The method is highly useful commercially because it permits the preparation from monomeric materials of a single high molecular weight polycarbonate, which may then undergo redistribution to yield a spectrum of lower molecular weight materials which can be custom designed depending on their intended use. A similar method, applicable to polyestercarbonates, is disclosed in copending, commonly owned application Ser. No. 08/373,805.
A broad spectrum of redistribution catalysts is disclosed in 5,414,057. Included are tetraalkylammonium hydroxides, tetraalkylammonium acetates, phosphines such as triphenylphosphine and organometallic compounds. The tetraalkylammonium hydroxides are often preferred because of their availability and particular suitability.
A tetraalkylammonium hydroxide which was initially chosen, by reason of its relatively low cost and its relatively low thermal stability, as particularly suitable was tetraethylammonium hydroxide. Low thermal stability was considered important since decomposition to the corresponding trialkylamine, a relatively volatile material which would be lost during extrusion, would afford a polycarbonate product which was expected to have a relatively low volatiles content. It was found, however, that triethylamine was present in the redistributed polycarbonate in amounts as high as 17 ppm (by weight) under typical plant-scale extrusion conditions. Amine levels on the order of 4 ppm or less, preferably 1 ppm or less, are desired.
It might be expected that the substitution of tetramethylammonium hydroxide, which upon thermal decomposition yields the more volatile trimethylamine, for tetraethylammonium hydroxide might satisfactorily solve this problem. Indeed, it was found that the trimethylamine level of a polycarbonate prepared by redistribution using tetramethylammonium hydroxide as a catalyst was very low, generally less than 500 ppb (0.5 ppm). However, even at this low level the trimethylamine imparts a distinct and unpleasant odor to the polycarbonate, rendering it unsuitable for many customer applications.
Therefore, it remains of importance to perform polycarbonate redistribution under conditions which afford a product of low volatiles content and without a detectable odor.